MICHAEL SZOENYI / SCIENCE PHOTO LIBRARY MICHAEL SZOENYI / SCIENCE PHOTO LIBRARY
Alpine bridge. This is the Ponte dei Salti, across the Verzasca River, near Lavertezzo, Switzerland. In front of the bridge, at left, is a point where orthogneiss and paragneiss metamorphic rocks have merged. Gneiss rocks form by metamorphosis, changes to pre-existing igneous or sedimentary rocks due to heat and pressure at depth. Igneous rocks form orthogneiss (light grey), while sedimentary rocks form paragneiss (reddish). The gneiss was lifted to the surface and exposed by erosion, both processes occurring as the Swiss Alps formed. Mixtures of the gneiss types, known as intrusions, help determine the relative ages of the rocks.
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